1. Field of the Invention
It is known that fire resistant papers can be produced from water-swellable inorganic minerals, in particular from dispersions of 2:1 layered silicates.
The 2:1 layered silicate minerals mica and vermiculite are made into flocs by the ion exchange of aqueous dispersions of the mineral lamellae. Flocculating exchange cations, such as those of U.S. Pat. Nos. 4,707,298, 4,877,484, and 4,239,519 (guanidinium, diamine and metal cations) are used to prepare the flocs. Fibrous materials and even fillers can be used and combined with the mineral.
After flocculation, dewatering and conventional paper-making technology allow the formation of paper with these mineral materials. The mineral ingredients give this paper extremely desirable flammability characteristics. Unfortunately, the papers have poor flexibility and poor internal adhesion between the flocculated ingredients.
In addition to this, the flocculating exchange cations that are used to form the silicate floc fail to form a readily drainable flocculated mixture. Thus, in preparing paper from 2:1 layered silicate minerals, processing difficulties are encountered. Although flocculation occurs, the silicate floc is very fine. The fine particles of floc make dewatering and sheet formation slow and difficult. It is also difficult to achieve a good distribution of the fibers throughout the floc.
Advantageously, the present invention provides paper which has good flexibility and has good adhesion between the wet-laid ingredients. In comparison to paper prepared with the cationic flocculants of the prior art, the paper prepared with the present method shows better flexibility and Z-direction strength. The papers of the instant invention also have an excellent fiber distribution.
The method described herein uses a dual, sequential flocculation system that advantageously provides rapid dewatering and sheet formation. This process, moreover, results in paper with excellent physical properties.